1. FIELD OF THE INVENTION
The present invention relates to a nozzle plate for spinning glass fibres used in a glass fibre manufacturing apparatus.
2. DESCRIPTION OF THE PRIOR ART
Glass fibres having diameter of 9 to 23 .mu.m are used in large quantities in form of glass cloth or glass mat as a basic material for FRP or printed circuit boards. These glass fibres are manufactured through a method in which molten glass is extruded through nozzle holes formed in a nozzle plate for spinning glass fibres and taken up with a predetermined speed by means of a take-up device. As partly shown in FIG. 3, the nozzle plate used here is formed with a nozzle 2 projecting from a flat plate portion 1, and the nozzle 2 is formed with a straight nozzle hole 3. The molten glass 4 flows out through the nozzle hole 3 and forms a cone 5 at the tip of the nozzle, and the cone is drawn downwards, thereby attenuating the glass flow and producing a glass fibre 6. Usually, the diameter d of the nozzle hole is in range of 1.0 mm to 2.5 mm, the length l of the straight portion of the nozzle hole is in range of 2 mm to 6 mm, the nozzle projecting length L is in range 3 mm to 6 mm, and the thickness t of the nozzle is about 0.3 mm.
Recently, by virtue of the advance in the glass fibre making art, it become possible to manufacture a glass fibre of about 3 .mu.m by use of a glass fibre spinning nozzle of a straight type of a prior art such as shown in FIG. 3. However, it is difficult to manufacture a glass fibre thinner than the above, which can be used as a spacer for a liquid crystal display.
The manufacturing difficulity is caused not only by the fact that the glass fibre itself is fine and easily broken, but also by problems included in the fibre attenuating method. For obtaining a thinner fibre, there are methods such as a method in which the diameter of the nozzle hole is decreased for decreasing the flow rate of the molten glass, another method in which the spinning speed (drawing speed) of the glass fibre is increased, and a combination of these methods.
Although the diameter of the fibre can be decreased by increasing the spinning speed, the fibre is frequently broken in spinning at a high speed, because the fibre is formed by drawing the cone of the molten glass at high speed, which molten glass flows out through the nozzle at low speed, and the strength of each fibre is low. Therefore, actually, the spinning speed can not be increased beyond 2,500 m/min and a limit exists in decreasing the diameter of the fibre manufactured by use of a nozzle hole having a rather greater diameter.
On the other hand, in the method in which the diameter of the nozzle hole is decreased for decreasing the flow rate of the molten glass, the molten glass is apt to be influenced by environmental conditions when the molten glass passes through the narrow nozzle. For example, there is caused a problem that the molten glass is rapidly cooled under the influence of the disturbed air around the nozzle and stops to flow out through the nozzle, thereby causing a breakdown of the glass fibre. When the glass fibre drawn from the nozzle hole has broken, the molten glass flowing out through the nozzle hole forms a drop at the tip of the nozzle, and when the drop grows up to a certain amount, the drop separates from the tip of the nozzle and falls down, thereby forming a cone of the molten glass at the tip of the nozzle and producing again a glass fibre continuous to the cone. In case of the nozzle of decreased hole diameter, the flow rate of the molten glass is small, and it takes a long time for the drop of the molten glass to grow to an amount sufficient to fall down. In this process, the molten glass 4 adheres even to the outer surface of the nozzle 2 as shown in FIG. 4, and does not easily fall down. In an extreme case, the molten glass extends over the outer surface of the nozzle up to the flat plate portion of the nozzle, and forms no drop, but adheres totally to the lower surface of the nozzle plate. As a result, the glass fibre spinning process can not be reopened. In case of reopening the glass fibre spinning process by high-handedly separating the molten glass adhered to the top of the nozzle from the nozzle by using such as tweezers, as shown in FIG. 5, the outer diameter of the cone 5, become greater due to the molten glass wetting the outer surface of the nozzle, and the molten glass stays on the outer peripheral portion of the cone. This staying molten glass is cooled by the atmospheric air, and the viscosity of the glass become greater. Since the molten glass thus having high viscosity mixes intermittently with the glass flow for forming a fibre, the cone become unstable and the fibre has a great fluctuation in its diameter and is apt to be easily broken.